The present invention relates generally to acquiring and displaying waveform data and more specifically to an apparatus and method for displaying waveform data having multiple segments with different sample spacings acquired using a measurement test instrument, such as an optical time domain reflectometer.
A traditional optical time domain reflectometer (OTDR) tests a fiber by acquiring a single waveform to represent an entire display trace of the fiber under test. The OTDR, under operator control, determines the portion of the test fiber to be examined, the pulse width of the optical pulses to be launched in to the test fiber, the sample density or spacing between acquired data points, the amount of averaging for each acquired data point, and the like. A series of optical pulses are launched into the test fiber. During the period between each test pulse, a return reflected optical signal is received, converted to an electrical signal and sampled in accordance with the preselected sample density. The acquired waveform data is stored in memory and further processed to locate and measure events on the test fiber. When using this technique, a trade off must be made between resolution and dynamic range. The resulting acquired waveform consists of a series of evenly spaced, constant sample spacing data points that are easily displayed on a display screen of the OTDR. Each data point of the acquired data displayed on the display screen is mapped to an x and a y coordinate of the display. Generally, the number of acquired data points is substantially greater than can be displayed on the display screen. The waveform trace displayed on the display screen is generated using some form of decimation where a constant number of data points are skipped between each data point that is displayed. A waveform, for the purposes of this specification, refers to the data collected from a single acquisition. This data has a single sample density and number of averages (i.e. the number of averages and the distance between samples is the same for each data point in the waveform). A trace refers to the traditional representation of the fiber that is presented to the user of the OTDR (i.e. the backscatter with Fresnel reflections and other events that appear on the OTDR display or hardcopy).
An example of the above described optical time domain reflectometer is the TFP2 Optical Time Domain Reflectometer manufactured and sold by Tektronix, Inc., Wilsonville, Oreg., the assignee of the present invention. A feature of the TFP2 OTDR is an expansion windowing system as described in U.S. Pat. No. 5,129,722. The expansion windowing system has first and second viewports with the first viewport displaying the waveform trace in a display area of the OTDR and the second viewport displaying a portion of the waveform trace from the first viewport in an expanded form. The second viewport is moveable in the first viewport as a function of a movable cursor in the display area, which intersects the waveform trace. Means are provided for varying the dimensions of the second viewport about the cursor/waveform intersection when the first viewport in the display area. As was previously described, the number of acquired data points is substantially greater than the number of data points that can be displayed on the display screen. The expansion windowing system takes advantage of this by being able to expand on a portion of the waveform trace in the first viewport and displaying that portion of the trace in the second viewport in greater detail by skipping fewer data points between the data points being mapped to the display screen.
An alternative method of acquiring waveform data in an OTDR type instrument is described in U.S. Pat. No. 5,155,439, assigned to the assignees of the present invention. The optical fault locator launches optical pulses into a test fiber at a first pulse width. The return reflected optical signal is converted to an electrical signal, digitized, stored and processed to locate anomalous events in the fiber. Any region of the test fiber having an anomalous event is reexamined using optical pulses having a pulse width optimized for the that region of the fiber. A symbolic display is used to indicate the location and type of event located in the fiber instead of the traditional waveform trace.
The trade-offs between OTDR resolution and dynamic range are optimized in the TFS3030 Mini-OTDR, manufactured and sold by Tektronix, Inc., Wilsonville, Oreg. The TFS3030 analyzes a fiber under test by acquiring several waveforms at different pulsewidths. Each waveform represents a different segment of the fiber, and each waveform is acquired using the shortest pulsewidth that is capable of testing the distances spanned by that segment of the fiber. Further, each waveform is acquired using a different sample density based on the pulsewidth. These waveforms are analyzed independently as they are acquired and the results are used to build a table of the events for the fiber, and to determine the acquisition parameters, e.g. pulsewidth and sample density, for acquiring the next waveform over the next segment of fiber. Each subsequent waveform segment acquisition overwrites the previously acquired waveform segment data. The resulting table of events for the various fiber segments is displayed on a display screen or stored for future reference. This method of testing the fiber has proven to be very accurate and efficient for locating events on a fiber. However, the TFS3030 does not use the acquired waveform segments to display a trace on the display screen representing the fiber under test.
One method of generating a wavform trace to go along with the table of events is to perform an additional acquisition to acquire a waveform that spans the entire fiber as was described for the TFP2 OTDR. While this would generate a trace of the fiber to go with the table, the data displayed to the user would not be the same data used to generate the table, and because a longer pulsewidth would be required to acquire the entire trace, it would not be as accurate as the individual waveform segment traces. The additional acquisition would also increase the time for testing the fiber and also increase the amount of memory necessary for storing waveform data.
A preferable type of OTDR would combine the advantages of the waveform segment acquisition technology of the TFS3030 and the waveform display capability of the TFP2. Such an OTDR should have the capability to test multimode optical fibers at 850 nm and 1300 nm and singlemode optical fiber at 1310 nm and 1550 nm. Below is a table showing representative parameters for acquiring singlemode waveform segments using the combined technology.
______________________________________ Singlemode Waveform Segments # of Segment Sample Start Stop Data # Pulsewidth Spacing Distance Distance Points ______________________________________ 1 2 m 0.5 m 0 km 2 km 4000 2 5 m 1 m 2 km 6 km 4000 3 20 m 5 m 6 km 26 km 4000 4 100-500 m 10 m 26 km 66 m 4000 5 1 km 20 m 66 km 106 km 2000 6 2 km 50 m 106 km 206 km 2000 Total # of Singlemode Data Points 20000 ______________________________________
As can be seem in the table, the sample density of segment 6 at 50 meters is 100 times that of segment 1 at 0.5 meters. One way to combine the segments so that they can be manipulated and displayed using the same sample spacing as in the TFP2 OTDR is to interpolate, the segments that have larger sample spacing. For example, make the segment with 1 meter sample spacing appear to have 0.5 meter sample spacing by interpolating between data points and adding a data point between existing samples. For segments with meter sample spacing, the interpolation would have to add 10 data points between samples. The following table shows the results of interpolation of the segments for the singlemode case.
______________________________________ Interpolation of Singlemode Waveform Segments Seg- # of Interpolate Interpolate ment Pulse- Sample Data d Sample d # of # width Spacing Points Spacing Data Points ______________________________________ 2 m 0.5 m 4000 0.5 m 4000 2 5 m 1 m 4000 0.5 m 8000 3 20 m 5 m 4000 0.5 m 40000 4 100-500 m 10 m 4000 0.5 m 80000 5 1 km 20 m 2000 0.5 m 80000 6 2 km 50 m 2000 0.5 m 200000 Total # of Interpolated Data Points 412000 ______________________________________
As can be seen in the above table, interpolating the waveform segments results in a very large number of data points, requiring more memory to store the data. This increases the cost of the instrument. There is also a reduction in the speed of the instrument in that additional time is required to perform the interpolation, storing and processing the data to generate the displayed trace.
Another way to combine segments so that they can be manipulated and displayed is to decimate the segments that have smaller sample spacing so that all segments have the same sample spacing. For example, make the segment with 10 meters sample spacing appear to have 20 meter sample spacing by removing every other sample. The following table shows the result of decimating the segments for the singlemode case.
______________________________________ Decimation of Singlemode Waveform Segments Deci- Seg- # of mated Decimated ment Pulse- Sample Data Sample # of # width Spacing Points Spacing Data Points ______________________________________ 1 2 m 0.5 m 4000 50 m 40 2 5 m 1 m 4000 50 m 80 3 20 m 5 m 4000 50 m 400 4 100-500 m 10 m 4000 50 m 800 5 1 km 20 m 2000 50 m 800 6 2 km 50 m 2000 50 m 2000 Total # of Decimated Data Points 4120 ______________________________________
In this case, the total number of data points does not present a problem. However, the result of the waveform decimation is that first waveform segment ends up with only 40 data points to cover a 2 kilometer span. This reduces the resolution significantly in a region that the resolution adds significant benefit, for example identifying jumpers used to connect the OTDR to the fiber under test. Not only is the resolution reduced, but it is possible that reflective events will be removed from the data during the decimation process. Further, there are not enough data points in the first two segments to provide an expanded waveform trace that provides any significant meaning. Therefore, the decimation process does not provide the desired results.
To avoid loosing information in the decimation process a combination of decimation and interpolation could be used. Using this technique, a target sample density is chosen so that no information would be lost in the decimation process. For the singlemode case above, a 2 meter sample density is chosen so that at least one data point exists for each pulse width of the shortest pulse width segment. The remaining segments are then decimated or interpolated to match this sample density. The results of this technique for the singlemode case is shown in the table below.
______________________________________ Interpolation & Decimation of Singlemode Waveform Segments Deci- Seg- # of mated Decimated ment Pulse- Sample Data Sample # of # width Spacing Points Spacing Data Points ______________________________________ 1 2 m 0.5 m 4000 2 m 1000 2 5 m 1 m 4000 2 m 2000 3 20 m 5 m 4000 2 m 10000 4 100-500 m 10 m 4000 2 m 20000 5 1 km 20 m 2000 2 m 20000 6 2 km 50 m 2000 2 m 50000 Total # of Data Points 103000 ______________________________________
The result is still a very large number of data points requiring more memory. There is also a reduction in the speed of the instrument in that additional time is required to perform the interpolation and decimation process, storing and processing the data to generate the displayed trace.
Combining waveform data in an OTDR is described in U.S. patent application Ser. No. 08/210,820, filed Mar. 18, 1994, entitled "An Optical measurement Instrument and Wide Dynamic Range Optical Receiver for Use Therein", assigned to the assignees of the present invention. The combining of waveforms is the result of acquiring waveform data sets over the total fiber under test using different optical power levels inputs to a low and a high sensitivity channel. In addition, the gain of the photosensitive device in the high sensitivity channel is varied. This produces waveform data sets that either have low sensitivity to low signal levels or saturate the optical receiver. The low sensitivity signal channel receives in the range of one to ten percent of the return optical signal and the high sensitivity channel receives approximately ninety to ninety-nine percent of the optical return signal. The sets of waveform data from the low and high sensitivity channels are acquired using the same pulse width and sample spacing and are correlated both vertically and horizontally and combined to produce a composite waveform where the waveform data from each data set is neither saturated nor in the electrical noise floor of the instrument.
What is needed is an apparatus and method for displaying acquired waveform segments in a measurement test instrument like an OTDR where the waveform segments have different sample densities. The method should use the existing acquired waveform segment data without having to interpolate or decimate the data thus requiring increased instrument memory and increased processing time.